1. Field of invention
This invention relates to a microscope objective lens system and more particularly, to an objective lens system for light within the visible wavelengths and that may be used in a plan-achromatic immersion type infinite microscope objective with a numerical aperture of approximately 1.25 to 1.30 or an achromat immersion type infinite microscope objective which also has a transmittance of greater than 50% for ultraviolet light with a wavelength of 340 nanometers (nm).
2. Description of the Related Art
For most known plan-achromatic or achromat immersion type infinite microscope objective lenses, a large number of lens elements are used for aberration correction. Generally, additional lens elements are added to an objective lens system in order to increase aberration correction. Thus, to produce a high quality objective lens, a large number of lens elements are generally used. An objective lens system that uses a large number of lens elements, however, is expensive due to the cost of the lens elements.
Where the desired objective lens system is to have a large aperture and strong magnification, a double Gauss lens system is generally included in the objective lens. The double Gauss system typically has four to six lenses (e.g., two doublets and additional singlets, although one doublet used in conjunction with a singlet may also be used) where there are two negative lens elements located with the concave lens surfaces facing each other. See, e.g., U.S. Pat. No. 5,132,845 (FIG. 1, r.sub.16, r.sub.17), to Suzuki, issued Jul. 21, 1992. In order to produce strong Petzval sum correction, the concave surfaces in the double Gauss system typically have a small radius of curvature. Unfortunately, because the radius of curvature is small, the negative lens elements in the double Gauss system are sensitive to even slight deviations in the curvature. Thus, the narrow tolerances of the double Gauss system require very precise fabrication. Consequently, an objective lens that uses a double Gauss system suffers from the disadvantages of being both difficult to fabricate and expensive.